1. Field of the Invention
The present invention relates to firearms, and more specifically to the rifling in the barrel of a firearm.
2. Description of Related Art
Gun barrels are typically made from a solid piece of steel or other metal. Traditionally, a center hole is cut through the center of a barrel using a specialized drill or other machine. The size of the initial hole is typically slightly smaller than the caliber of the gun barrel. Then grooves twisting around the inside of the barrel are formed to create a rifling pattern. The rifling pattern on the inside of the barrel imparts spin on the bullet or other projectile being fired. The rifling spins the projectile about its long axis, thus stabilizing the flight of the bullet and improving its aerodynamic stability and accuracy. The rifling is provided on inside of the barrel as equally spaced lands separated by grooves along the barrel circumference. Conventional rifle barrels typically have several lands which are separated by grooves within the rifle barrel. The grooves are made by cutting material out from the inside of the barrel, leaving a land (ridge) between each pair of grooves. The grooves are machined around the inside of the barrel in spiral twist pattern. The lands are designed to maintain contact with the sides of the bullet as it is projected down the barrel, thus imparting a spin on the bullet as it leaves the muzzle of the rifle.
FIGS. 1A and 1B depict two types of conventional rifling. The most common type of conventional rifling, as shown in view 100 of FIG. 1A, consists of lands and grooves with relatively sharp edges that bite into the surface of the bullet. View 120 shows further detail on land 106. The land top 106 and land sides 104 and 108 are sometimes at nearly right angles to each other, as depicted in view 100. In other conventional embodiments, as depicted in view 120, the land top 106 may be at an angle with the sides 112 and 114 greater than a right angle. In any event, the sides of the lands in conventional implementations are Recently, however, a newer type of polygonal rifling has been used in handguns. Polygonal rifling is characterized by lands with a more rounded, curvilinear polygon shaped profile. Proponents of polygonal rifling point to the higher muzzle velocities and greater accuracy achievable with polygonal rifling. Moreover, barrels with polygonal rifling tend to last longer than barrels with conventional sharp edged rifling due to the reduced friction between the bullet and the rifle barrel.
Rifling is characterized by a twist rate which affect the rate of spin imparted to a bullet. The twist rate is defined as the distance a rifling land takes to complete one full revolution within the barrel. Twist rates vary based on the size, shape and weight of the projectile being fired. A shorter distance twist provides a faster twist, producing a higher spin rate on the projectile. A twist rate of 1 turn in 8 inches (1:8 inches) is faster than a 1:12 inch twist rate. In general, longer twist rate barrels are used with larger diameter, shorter bullets (e.g., spherical lead balls) while relatively longer, small diameter bullets are typically fired through shorter (faster) twist rate barrels. For example, a large diameter muzzle-loading rifle that shoots spherical lead balls might have a low twist rate of 1:48 inches, that is, 1 turn in 48 inches. At the other extreme, pistols—e.g., 9 mm, .357 and .40 caliber—often have a twist rate of approximately 1:10 inches.
In regards to caliber, it should be noted that for a given caliber there are often several different types of rifle, each of which has different rifling characteristics. For example, rifles in the 30 caliber family include .30-06 Springfield, .30-30 Winchester, 308 Norma, 308 Winchester, 300 Winchester Mag, and others. All of these rifles shoot 30 caliber bullets (0.308 inch diameter bullets) and are 30 caliber as measured from the top of one land to the top of the land on the opposite side of the bore. The “08” in a 308 Winchester means that each land has a four thousandths of an inch groove next to it (two grooves of four thousandths of an inch give us the “08”). In this way, the rifling characteristics tend to be somewhat different in each different type of 30 caliber weapon. For example, the number of the grooves or the groove profile or land shape often differs from one model of weapon to the next.